Air guns, as used herein, are sound sources for marine seismic exploration. The operating components of air guns include a firing chamber holding a charge of gas under high pressure, a two-piston shuttle assembly having a firing piston which retains the charge of pressurized gas within the firing chamber, and an operating piston positioned within an operating chamber where the highly pressurized gas acts against the operating piston to maintain the shuttle assembly in a closed position until firing. A shuttle assembly interconnects the two pistons and provides for pressurized gas to flow from the operating chamber through the shaft of the shuttle assembly to charge the firing chamber. The air gun is triggered using a solenoid operated valve to release pressurized air into the operating chamber actuating the shuttle assembly to cause an abrupt discharge of pressurized air from the firing chamber through discharge ports and directly into the surrounding water creating an acoustic pulse.
Air guns as deep penetration sound sources, produce output frequencies generally between 10 Hz to about 1200 Hz, to identify subsurface geologic layers and define the subsurface structure. Air guns of the prior art are typically fired at compressed air pressures in the range of 2000 psi to 3000 psi. These conventional air guns produce an initial pulse or primary pressure pulse typically of about 1.5 milliseconds (ms) of rise time to reach peak pressure. This very fast rise time produces an abundance of high frequency components in the primary pressure pulse which are outside of the frequency range of interest and therefore may not be beneficial in the identification of subsurface geological structures. High frequencies may further be a source of damage to marine mammals and fish as well as disturbing the habitats and well-being of marine life. There is recently mounting pressure on the exploration industry to eliminate these undesirable high frequencies from the pulses of the air guns used in air gun arrays. The inventor hereof has additional patents such as U.S. Pat. Nos. 3,379,273, 4,038,630, 4,271,924, 4,599,712, 4,712,202, 4,779,245, 5,432,757, and 8,223,591. There are also some other inventors in the same field such as Fiske, U.S. Pat. No. 4,757,482, Mayzes, U.S. Pat. No. 5,315,917, Jensen, U.S. Pat. No. 7,269,099, Hopperstad, U.S. Pat. No. 7,321,527 and others.